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Teaching A2 Chemistry Practical Skills
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Appendix 2
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29. Preparation of poly(pyrrole) (a conducting polymer)
Student sheet
In this exercise, you are going to make a conducting polymer. You will then, using a circuit of
your own devising, test its conductivity.
Intended lesson outcomes
By the end of this exercise you should be able to:
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handle hazardous materials safely
work with delicacy and precision
measure the conductivity of a polymer
understand the polymerisation process and the concept of a conducting polymer
Background information
Electrical conductors work by allowing a flow of electrons though them. This requires that
delocalised (mobile) electrons are present in the conductor. Metals conduct because they
easily lose their outer shell electrons to form what may be described as a ‘sea of delocalised
electrons’. Graphite conducts owing to its bonding/structure; the carbon atoms form an
extended system of π bonds which are delocalised across the whole plane of a graphite sheet.
Consider the bonding in benzene. It may be thought of as having alternate double (π bonds)
and single bonds (σ bonds). However, the benzene ring does not behave like an alkene. The
reason is that an electron from each carbon atom is delocalised. The bonding in the ring is,
therefore, described as being delocalised, and the mobile electrons are called delocalised
electrons. Molecules with alternating double and single bonds are described as having a
conjugated system of bonding. In a conjugated system, some electrons are delocalised and
so can move freely throughout the conjugated system.
The structure of pyrrole, C4H5N, is shown below. You will notice that the two double bonds are
conjugated. Also, there is a lone pair of electrons on the nitrogen atom that can also join in the
conjugation so that together they provide six delocalised electrons, similar to those in benzene;
see the diagrams below.
N
H
N
H
Under appropriate conditions, pyrrole polymerises to form poly(pyrrole), the structure of which
is shown below.
H
N
H
N
N
H
H
N
H
N
N
H
N
H
N
H
In both poly(pyrrole) and poly(ethyne), (CH=CH)n , the delocalisation extends along the
whole length of the polymer chain. However, in order to make such polymer chains conductors,
electrons have to be added (reduction) or removed (oxidation) from the conjugated system.
Electrons can then move along the polymer chain, through the conjugated system just as they
do through graphite or a metal.
In this exercise, you will make some poly(pyrrole) by growing it on a nickel anode as a thin
film. You will then remove this film by carefully pealing it off the anode, and test its conducting
properties.
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Teaching A2 Chemistry Practical Skills
Appendix 2
Safety
There are potentially hazardous substances involved in this exercise. You must follow all
health and safety instructions given to you by your teacher. Materials data safety sheets should
be consulted so that the correct action can be taken in event of a spillage and/or accident.
You must wear eye protection throughout this experiment
Pyrrole is toxic
Sodium 4-methylbenzenesulphonate is irritant and harmful
Propanone is highly flammable
12 V
Variable
resistor
0–5000 Ω
A
Cathode
(Copper)
Anode
(Nickel)
Pyrrole
solution
Procedure
1.
Copper electrode: Clean the piece of copper supplied with wire wool or emery paper and
then rinse it with distilled water. Loop it over the side of the 250 cm3 beaker so that it
reaches the bottom.
2.
Nickel electrode: Clean the flat end of a nickel spatula with a non-abrasive metal cleaner
such as 'Brasso' (you will only be able to peel the polymer off if the nickel electrode is very
clean and smooth). Rinse thoroughly with distilled water, and then with propanone. Allow
the spatula to dry in the air.
3.
Do not touch the flat part of the spatula with your fingers once you have washed it.
4.
Work in a fume cupboard. Using a teat pipette, drip about 0.4 g of pyrrole into a 250 cm3
conical flask (CARE: irritant). If you spill any pyrrole on your hands, wash it off with lots
of water.
5.
6.
Add 100 cm3 of sodium 4-methylbenzenesulphonate solution (concentration
0.100 mol dm–3) to the pyrrole. Swirl the flask thoroughly until the pyrrole has dissolved.
7.
Pour the pyrrole solution into the 250 cm3 beaker and set up the circuit as shown in the
diagram.
8.
Start with a low current and gradually increase it to 30 mA.
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Teaching A2 Chemistry Practical Skills
Appendix 2
9.
The nickel electrode should turn black within seconds and bubbles of hydrogen will form at
the copper cathode.
10. Continue passing a current for about 45 minutes.
11. Remove the nickel electrode and wash it with water. Carefully peel off the poly(pyrrole) film
in one piece using a scalpel or razor blade.
Testing the conductivity of poly(pyrrole)
1.
Place your film on a glass slide. Fold the film in half and half again (four thicknesses) to
reduce the possibility of it burning out when a current is passed through it.
2.
Using crocodile clips, connect the film into a simple series circuit containing a 1.5 V bulb
and a variable 12 V d.c. supply. Initially, place the crocodile clips about 5 cm apart and
start with a p.d. of 2 V. Gradually increase the p.d. and then move the crocodile clips closer
together. You should find that when the p.d. is 12 V and the crocodile clips are about 1 cm
apart, the bulb will light.
Question
Suggest, with explanation, how the conductivity of poly(pyrrole) compares with that of copper,
and with polymers such as poly(ethene) or nylon.
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Appendix 2
29. The preparation of poly(pyrrole) (a conducting polymer)
Teachers’ Notes
The exercise provides a novel way of preparing a polymer which, once formed, has unusual
properties. To work properly, care and precision are required.
Intended learning outcomes
Please see the Student Sheet
A suggested approach
This is a relatively simple practical exercise but it does require great care and patience if it is to
work properly. The chemicals used must be handled with care but the exercise is not beyond
the capabilities of A level students.
The polymer is formed at the anode of an electrolytic cell, with hydrogen gas being evolved at
the cathode. You may wish to revise the theory of electrolysis with your students prior to
starting the exercise.
More able students may well deduce that H+ ions are being produced at the anode, along with
the polymer, and that hydrogen atoms are being removed from the 2- and 4- positions on the
pyrrole ring. Putting these two ideas together could well result in an interesting discussion of
the polymerisation process.
The polymer film is quite fragile and will need to be handled carefully. Students should aim to
remove it in one piece and then transfer it to a microscope slide for support. The crocodile clips
used to connect the film into the circuit will also need to be carefully positioned if the film is not
to be damaged.
It is normal for the teacher to trial experiments before using them with a class of students. The
wise teacher will certainly trial this exercise!
Answers to question
The conductivity of the polymer is:
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lower than that of copper (copper doesn’t need 12 V to light a 1.5 V bulb, so must have a
much higher concentration of delocalised electrons)
greater than Terylene and nylon (they are non-conductors due to the absence of
delocalised electrons)
Technical information
Requirements per student/group
Apparatus
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copper foil (1 cm wide and long enough to be folded over the edge of a beaker and still
reach to the bottom)
nickel spatula
one 250 cm3 beaker
one 250 cm3 conical flask
0–5000 Ω variable resistor
12 V d.c. supply (2 V steps)
ammeter range 0–30 mA
1.5 V light bulb and holder
teat pipette
razor blades or equivalent
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© University of Cambridge International Examinations 2006
Teaching A2 Chemistry Practical Skills
Appendix 2
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microscope slides
access to fume cupboard
crocodile clips
wires
Materials
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pyrrole (0.4 g)
100 cm3 of sodium 4-methylbenzenesulphonate (p-toluenesulphonic acid sodium salt)
solution of concentration 0.10 mol dm–3
propanone (20 cm3)
wire wool or emery paper
'Brasso' metal polish
small pieces of cloth
paper towels
Safety
The main points are included on the Student Sheet. However:
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It is essential that a risk assessment is carried out before a decision is taken to go ahead
with this exercise. MSDS sheets should be consulted so that the correct action can be
taken in event of a spillage and/or accident.
It must be made clear to students that potentially hazardous materials are in use and that
precautions are needed to minimise the risk to themselves and to others.
Your MUST be prepared to intervene if a student seems to be unsure of a procedure, or is
performing an unsafe operation.
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© University of Cambridge International Examinations 2006